If you're looking to screw them that means doing at least -10 before you see a difference. The +chr song will give 2-4 charisma at hero (depending on rep) and almost all bard instruments and bard-specific eq has +chr built around other sweet benefits. This means a well equipped (romantic?) bard will have +10 charisma going for them easily. More if they get items like the dragon coat (+4) or cloak of patches (+3). Even if you do hit with a faceslash or skunk spray - there really isn't enough to stack on.

Conjurers are far more vulnerable, as their eq sometimes does have +chr but they don't have any inherent way of adding to it, and will tend to go for elite mage gear rather than the conjurer-specific stuff.

Yhorian